Reload before you fell in, then activate the pit (just step slightly into the room to do so. From the top, press the buttons down below (for some reason, you can reach them from the top.) Fill the pit with scorpions (20 pushes on either the north, east, or west buttons, but stay away from the south button, as it raises a pillar that you can't get back to the pit from.) Then firebomb them (Burning Haze or Firestorm). You get around 750 to 1000 XP per scorpion, which equates to around 15000 to 20000 total XP. Now repeat the action as long as you like.